1. Field of the Invention
The present invention relates to a communication recovering system, and more particularly to a communication recovering system for a wavelength division multiplexed passive optical network (WDM PON) which is capable of being implemented with only a single optical fiber using a periodically arrayed waveguide grating (AWG) or protecting/recovering fault of optical fiber by minimizing optical fiber loss, although double optical fibers are used.
2. Description of the Related Art
Recently, as various data services and multimedia services are rapidly increased through the Internet, a large amount of transmission capacity is needed in a subscriber network. In order to comply with such requirement, a wavelength division multiplexed passive optical network (WDM PON) has attracted considerable attention. Here, the WDM PON is capable of providing a wide variety of services as optical signals whose wavelengths are different from each other are provided to each subscriber, and a plurality of optical signals are implemented via wave division multiplexing to be transmitted using a single optical fiber. Such a WDM PON has advantages in that, as outdoor networks are implemented with passive elements to comply with fast transmission of information, networks can be easily installed, maintained and administered, and also extension and security are high. Furthermore, the WDM PON can provide various kinds of services according to wavelengths.
Although such a WDM PON has the above advantages, when substantially implementing the WDM PON, fist of all, economical efficiency therefor must be considered. For that, various light sources, such as, a wavelength-selectivity distributed feedback laser, an injection-locked Fabry-Perot laser, and a spectrum-sliced light emitting diode, etc., have been researched.
Another consideration factor is network reliability when the WDM PON is substantially implemented. Since the WDM PON has a high transmission speed for optical signals compared with prior art subscriber networks, when communication outage occurs due to cutting or errors of optical fibers or optical components, etc., effects thereof are very serious. Generally, when optical components are installed in a sealed container or stable place, occurrence of errors can be reduced, or when the same components are additionally positioned therein, errors can be more effectively prevented. However, since the optical fibers are constructed as outdoor networks, they may be cut at any place and at any time by unexpected causes, such as, road construction, pipe works, and ground collapse, etc. Especially, when a wavelength division multiplexed optical subscriber network suffers structural breakdown, it is impossible to recover the network, thereby weakening reliability of the optical subscriber network.
FIG. 1a is a view illustrating a system for a wavelength division multiplexed passive optical network (WDM PON) of a double star architecture of the prior art.
Referring to FIG. 1a, a central office, CO, is connected to a plurality of remote nodes, RN, through one or more optical fibers. Optical signals are divided into different wavelengths in each remote node including a passive wavelength division demultiplexer, and then are transmitted to each optical network unit, ONU, through optical fibers.
The central office includes transmitters, receivers, wavelength multiplexers and demultiplexers. Optical signals whose wavelengths are different from each other are multiplexed by arrayed waveguide gratings (AWG) and are then transmitted to remote notes via optical fibers. Here, the optical signals are transmitted from a plurality of transmitters. Another AWG in the remote node demulplexes multiplexed signals to send optical signals whose wavelengths are different from each other to each optical network unit. A receiver in the optical network unit transforms the optical signals into electrical signals to perform communication.
Conversely, optical signals transmitted from the transmitters in the optical network units are multiplexed by the same AWG in the remote nodes, and are then transmitted to upstream. Afterwards, the transmitted signals are demultiplexed in the central office and are then received by the receiver.
Accordingly, since a plurality of multiplexed WDM signals are simultaneously transmitted between the central office and the remote nodes, there may be risk that a plurality of optical signals are lost due to optical fiber fault, which is referred to as communication outage. Also, since only a few of specific wavelength optical signals are transmitted between the remote nodes and optical network units, there may be risk that the specific wavelength signals are lost due to optical fiber fault. In order to prevent such optical fiber faults, another optical fiber installed at a detouring optical fiber must be secured. When installing the detouring optical fiber, economical efficiency, efficiency, network recovery time, etc. must be considered, and also the network must be designed such that loss of optical signals is minimized.
FIG. 1b is a view illustrating an optical fiber doubling technology to recover optical fiber fault between a central office and remote nodes in the system of the WDM PON according to FIG. 1a. 
Referring to FIG. 1b, a plurality of transmitters Tx and receivers Rx in a central office CO perform communication with transmitters and receivers in each subscriber based on optical signals which are allocated unique wavelengths. The optical fibers having a plurality of wavelengths perform wavelength division multiplexing or demultiplexing via an AWG having a 1×N structure in the central office and the remote nodes. Here, the AWG in the central office is connected to a 1×2 optical switch (1×2 OS), and the AWG in the remote node is connected to a 1×2 star coupler. The 1×2 OS and the 1×2 star coupler are connected to a working fiber and a protection fiber, respectively. Here, the working fiber is operated in a normal state. The protection fiber is operated in a state wherein the working fiber is not operated, and installed therein to form a detouring path. Therefore, the 1×2 OS is connected to the working fiber in a normal state to perform communication between each optical network unit and the central office. When there is a fault at the working fiber, an optical fiber fault monitor, M, detects the fault and rapidly changes the state of the 1×2 OS such that current communication is maintained through the protection fiber. After changing the state of the optical switch, optical signals are recovered through the protection fiber such that communication can be maintained without stoppage. Although such an optical fiber doubling technology requires an additional optical fiber to form a detouring path, it has still attracted considerable attention since any fault generated in the optical fiber can be rapidly recovered. Also, as the remote nodes are implemented with passive elements, costs for maintaining and repairing the networks can be minimized.
However, when the method of FIG. 1b is applied thereto, it has a disadvantage in that performance margin of signals is reduced by more than 3 dB due to optical loss of the star coupler. Also, it has drawbacks in that fault of optical fiber between the remote nodes and the optical network units cannot be recovered. [REFERENCE: A. J. Phillips et al., “Redundancy strategies for a high splitting optically amplified passive optical network,” J. Lightwave Technol., February 2001]
On the other hand, although specific methods to implement such an optical fiber doubling technology have been proposed [REFERENCE: A. H. Gnauck, et al., “Reliable architecture for fiber-based broadband local access networks,” US Statutory invention registration, US H2075H, Aug. 5, 2003], when a network is entirely connected using optical fibers, since a plurality of protection fibers and AWGs are required, the networks are extremely complicated. Also the networks are not cost-effective. On the other hand, when the number of AWGs is reduced, since usage efficiency for leads of the AWG lead becomes less than 50% and signals are separated using a star coupler, it has disadvantages in that loss of the optical fiber is largely increased. Accordingly, optical fiber protection devices in the WDM PON, which are capable of cost-effectively and efficiently using the networks and minimizing optical fiber loss, are needed.